Graphene oxide benefit is a wash out

U. WARWICK (US) — Graphene oxide—long heralded because it is easier to make in bulk quantities at a lower cost than pure graphene—appears to have a downside: it’s solubility literally comes out in the wash.

Researchers made the discovery when treating the graphene oxide with sodium hydroxide (NaOH) in an attempt to increase the usefulness of the oxygen containing functional groups believed to be bound to the graphene.

What they got seemed to make things worse rather than better, says Jonathan Rourke, chemist at the University of Warwick. Indeed at high enough concentrations of NaOH, Rourke was left with a black suspension.

Previous research had shown that oxidation debris adheres to carbon nanotubes but the weak nature of the connection of the debris to the carbon nanotubes means that a wash with a base can simply remove the oxidative debris.

Experiments show that in that particular case oxidative debris was found to make up almost a quarter of the mass of the “oxidized carbon nanotubes.” The researchers felt a similar process could be happening in the graphene oxide they were studying.

The results may also help explain the inordinately high levels of oxygen found in graphene oxide. Chemists were already struggling to identify enough plausible carbon to oxygen bonds to accommodate the amounts of oxygen believed to form part of graphene oxide.

On centrifuging the black liquid the researchers were left with a pile of black powder that turned out to be graphene oxide that may once have been soluble before the application of the base but which refused to show any significant sign of being easily soluble again in its current state.

The black material was found to be similar to graphene itself; in particular it was shown to consist of large sheets of electrically conducting carbon atoms, unlike the insulating “graphene oxide.”

The remaining liquid was also dried to give a white powder that the Warwick researchers showed contained the “oxidative debris” shown to be made up exclusively of small, low molecular weight compounds (i.e. less than 100 atoms).

The graphene oxide recovered from the washing process formed about 64 percent of the mass of the “graphene oxide” at the start of the process. The recovered OD or oxidative debris formed at least 30 percent of the weight of the mass of the original “graphene oxide.”

Rourke and physicist Neil Wilson believe this shows that much of the oxygen that was believed to be closely bonded to the carbon in the graphene oxide was actually not bonded at all but simply lying on top of the graphene sheets, loosely connected to them as “oxidative debris” that contained a large quantity of oxygen that came out in the wash when the graphene oxide was treated with sodium hydroxide.

“Our results suggest that models for the structure of graphene oxide need revisiting,” the researchers write.

“These results have important implications for the synthesis and application of chemically modified graphene particularly where direct covalent functionalization of the graphene lattice is required.”